Intelligent communications within a local-area network (LAN) of computers and associated peripherals and terminals depends on both the type of hardware utilized to effect the communications link and also on the modes of behavior, or protocols. A number of different techniques are utilized to effect the communications link and transfer information over the network. These techniques generally utilize a contention accessing scheme, a token-passing accessing scheme or a ring network. Each of these schemes utilizes a common communications link with an intelligent controller to control the data flow between units. A disadvantage to this is that the communication is not real time and requires data to be placed in packets and then transmitted when the communications link is free. Each of these packets, termed Protocol Data Units (PDU), contain some type of address field and control field. These fields designate the destination service access point and also the originating peripheral device. A disadvantage to these types of systems is that they do not operate in real time.
One type of scheme for accessing data packets from a local network is carrier-sense multiple access with collision detection (CSMA-CD). The CSMA-CD is a random access scheme in which a node competes with other nodes for the use of the network (multiple access). Before a node can transfer the message, it must first listen to the desired channel to make sure that it is not busy. The node recognizes a busy channel by detecting the presence of a carrier frequency. If busy, the node cannot transmit until the channel is clear. Once transmission starts, the node must monitor the channel again to make sure that no other nodes are transmitting on the channel at the same time. If messages do collide, the transmission is aborted and the node waits or backs off for a random period of time before it attempts retransmission.
The CSMA-CD system is defined in the Institute of Electrical and Electronics Engineers (IEEE) Standard IEEE-802. This standard applies to baseband coaxial-cable transmission, single channel baseband or multichannel broadband modes. Depending upon the type of mode utilized, the signalling rate will vary. However, all the interfaces used with the network must support a given signal rate. These interfaces are defined by standards such as RS-442, RS-232-C and RS-449 interfaces which are standard interfaces of the EIA (Electronic Industries Association).
In view of the above disadvantages, there exists a need for a communications network that interconnects peripherals in a communications network for real time communication and overcomes speed limitations of standard interface devices.